Apparatus for measuring moisture



Nov. 7, 1944. D. BAVER ET AL I APPARATUS FOR MEASURING MOISTURE Original Filed June 12. 1940 2 Sheets-Sheet 1 Cueuaueo LOAN MmsT 0 RF. EquwALENT x FIRST 5ER\F.S

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L. D. BAVER ET AL APPARATUS FOR MEASURING MOISTURE Nov. 7, 1944.

Original Filed June 12, 1940 2 Sheets-Sheet 2 Invcnmrs LEONARD l1 BHVER v B mtlbnn y nttomus Patented Nov. 7, 1944 APPARATUS FOR MEASURING MOISTURE Leonard D. Bayer, Raleigh, N. 0., and Byron T.

Shaw, Columbus, Ohio, assignors to The Ohio State University Research Foundation, Columbus, Ohio, a corporation of Ohio Original application June 12, 1940, Serial No.

340,170. Divided and this application December 31, 1940, Serial No. 372,476

2 Claims.

Our invention relates to an apparatus for measuring moisture.

It is the object of our invention to measure moisture in materials, particularly in such materials as soils, soaps. grains, flour, clays, paper and the like. I

It is theobject of our invention to provide an "apparatus for'determining heat conductivity in materials as an index of moisture in the materials.

It is a further object to provide means of measuring moisture in such a manner that salts in solution and the material to be measured, as in soils, will not affect the measurement.

It is a further object to provide an apparatus for measuring changes in the heat conductivity of materials in various moisture contents.

It is an object to use heat conductivity as an index of the moisture contentof materials.

Heretofore electrical conductivity methods of measuring moistures in soils and other materials have failed because the conductivity of the soil at a given moisture content varies greatly with changes in the salt concentration of the soil solution. We have discovered thatheat conductivity is not materially aiiected by the presence of ions in solution since rather large changes in the-concentration of a dilute salt solution have very little influence on the thermal conductivity.

We have further discovered that the heat conductivity of a dry porous medium such as soil must of necessity be low as the solid materials make only point contacts. The area. for continuous heat flow through soil materials is very small.

A negligible amount of the heat is conducted by the air in the pores, since air is a much poorer conductor than the soil solids. As water is added to the soil, the area through which heat can flow will increase tremendously since the water will form wedges around 'the points of contact.

Water is not as good a conductor of heat as the solid soil material, but it is a far better conductor than air.

We, therefore, have found that the heat conductivity of a soil or other materials would increase with its moisture content; and this is in-- It is an object of this invention to provide various apparatus of different types for practicing the method by which heat conductivity can be used as an index of soil or other material moisture since the conductivity of the materials increase with their moisture content.

It will be understood that the term soil is used generically and it is understood to cover any materials capable of being measured as indicated herein.

This application is a division of my copending application Serial No. 340,170, filed June 12, 1940.

Figure 4 is a view, in section, of a modified form of the heat measuring device in which the materials whose moisture is to be measured pass through the center of the coil instead of surrounding the outside of it. Figure 5 is a chart indicating a moisture cahbration curve for a typical soil such as ,chenango loam.

Figure 6 is an isometric view of the wiring arrangement of the back of a typical measuring instrument.

Figure '7 is a diagrammatic view of a wiring arrangement of an alternative form to that shown in Figure 1 for the practice of the method by which heat conductivity may be used as an index of moisture content.

.Referring to the drawings in detail:

Figure 1 shows an instrument operated on the basis of heat conductivity by means of which it is possible to follow the moisture changes in the soil or other materials over substantially the entire range of the moisture content thereof. It can be seen that the arrangement is that of a Wheatstone bridge. The sensitivity of the in strument is very nearly proportionate to the cube of the current flowing through the bridge.

It is very important therefore to keep the current constant during the course of a determination.

Before describing the operation of this instrument by which it is first calibrated and then operated to determine moisture content,'we proceed to the description of the construction of the instrument in Figure 1. I

1 The apparatus An arm l of the Wheatstone bridge is extended into a coil 2 which is inserted into the materials to be measured. The coil 2 is mounted around an insulation core 3 as in Figures 1, 2 and 3, or is mounted within an insulation core 4 as in Figure 4. This coil consists in a typical instrument of No. 40 enameled copper wire wound on a 6-mm. glass or Bakelite tubing designated 3. One end of the coil 2 is connected by the leg portion to the terminal Ii.

It should be noted that in the construction of this coil 2 that it is connected at its respective ends by the wires la and 5a which are leads of relatively large copper wire that go through isfactory.

the walls of the insulation tubing 3 and are soldered to the fine wire 2 on the exterior of the tube. The connections are enameled and the tube 'is sealed water tight by the seal 6. The

resistance of this arm is about 7 ohms at zero degree. The leg I of the Wheatstone bridge contains a coil of manganin wire of about 7' ohms as at 8. The leg 9 contains a similar coil in of about 200 ohms. The leg ll contains a variable resistance generally designated l2 which is in parallel with a manganin resistance of about 200 ohms.

This bridge is provided with a bias circuit 13 in which is located a microammeter M. This completes the measuring circuit.

The calibrating circuit consists of the wire 15, a switch IS, a wire H, a variable resistance 18, a wire 19, a six volt battery 20, the wire Ziand ammeter 22 in the battery circuit which is connected by the wire 23 to the terminal 25 of the legs 'I and H of the. bridge.

It will be observed in.this arrangement that the resistance of the coil 2 increases with its temperature, but the other three resistances being made of manganin wire are not influenced by temperature changes. In balancing the bridge a small current of about 0.02 ampere is used so that the heat developed in the coil 2 can be dissipated in the soil without causing the temperature to rise. The circuit is made and broken immediately. The variable resistance 12 is varied until no current flows through the microammeter. The bridge remains in balance as long as the temperature of the coil 2 remains constant. When the temperature of the soil or other material surrounding the coil 2 changes, a new value of resistance 12 is obtained at the balanced condition. Thus, it can be seen that the value of resistance [2 at balance is an index of the soil temperature; and, in fact, if a ourrent of 0.1 ampere or more is allowed to flow through the bridge continuously after a balance has been obtained, the bridge is thrown out of balance allowing a current to flow through the microammeter M. This is due to the fact that heat is being developed at the coil 2 faster than it can be conducted away by the soil, so that, as a result, the temperature of the coil 2 rises. This temperature continues to rise until it reaches a point at which heat is lost as fast as it is gained. The current through the microammeter It increases with the. temperature of coil 2. The magnitude of the current over equilibrium reflects the increase in temperature. It can be seen that the heat conductivity ofthe soil is the factor that controls the temperature rise. If the conductivity is low the temperature rise will 'be large, and if the conductivity is high the temperature rise will be small. Since the heat conductivity of a soil or other material increases with its moisture content, the size'of the current through the microammeter is an index of the amount of moisture in the soil. It is not necessary to keep the current flowing until the equilibrium condition has been reached. A microammeter reading taken at any arbitrary time after the current has been turned. on serves equally as well.

In order to calibrate the instrument and check the results, the following procedure is found satc'alibmtion methods a time as it was felt that the excess water had drained away the leads from coil 2 were connected to the bridge proper and a balance obtained. A current of 0.4 ampere was then passed through the bridge for one minute at which time the current through the microammeter was read. The leads were then disconnected and. the entire soil assembly, excluding the pasteboard cover, weighed. Since the weight of the soil was known as well as the weight of the container and the measuring element, it was possible to calculate the moisture content of the soil at the time the reading was taken. This particular type or container was chosen so that only a negligible amount of water would be in its walls and the excess weight could be assumed to be water actually in the soil. It was necessary to have porous walls so that evaporation could proceed on all sides and thus keep the moisture distribution in some semblance of uniformity. After weighing, the soil was allowed to stand uncovered until the following morning. During this time interval, water was lost by evaporation. The cover was then placed over the container in order to slow up evaporation. A new reading was obtained sometime during the afternoon and then the whole process was repeated for as many days as was necessary to bring the soil to the air-dry state. At this time the soil was resaturated and the whole process repeated. It is not to be inferred that readings can only be taken once each day, for onmany occasions readings were taken as often as every 20 minutes. The procedure described was merely a routine that fitted into the days work.

The result of this method of calibration is shown in the chart, Figure 5, where the vertical column indicates the amount of direct microamperes and the horizontal column is graduated in degrees of per cent of moisture. This curve taken in connection with chenango soil and the mixture determination thereof is a result of the calibration tests which are indicated by cross marks as the first series, while the circles indicating the second series of tests indicate the actual soil tests. It will be readily appreciated that the microammeter may be designed to read in percentages of moisture when suitably so callbrated.

While the relationship between heat conductivity and moisture content are distinct for each soil, yet all of the curves that we have made have some features in common. The increase in heat conductivity (a decrease in the detector current) with moisture is rather gradual in the high moisture ranges and very rapid in the low moisture range.

In field measurements, the coil 2 can be left in the soil or other materials permanently, if desired, while the remaining parts of the apparatus may be placed in a portable box as indicated in Figure 6. This apparatus can be plugged in at the time of reading. By such an arrangement it is possible to have a large number of elements such as the coils 2 placed at various locations which can be read with one bridge.

It has been stated that this method depends upon the changing heat conductivity of the soil with moisture, but it must be added that the changing heat capacity of the soil-water system also has a small effect upon the readings. By heat capacity we mean the amount of heat necessary to change a material one degree in temperature. As the soil decreases in moisture,the conductivity and capacity are both lowered. In addition to the greater rise in temperature of the soil element, due to' the decreased conductivity in the lower moisture ranges, a further increase in temperature will be caused since a smaller amount of the heat escaping will be required to produce a given change in the temperature of the soil in the neighborhood of the element. This effect of heat capacity is of only secondary importance, but since it is in the same direction as the conductivity effect it helps to increase the sensitivity of the method.

' The first criticism of any method of moisture measurement based on applying heat is likely to be predicated on the fact that moisture will move away from the point at which heat is applied. The fact that moisture moves cannot be denied; however, the amount of movement depends upon both the temperature of the heat source (relative to the soil) and the length of time the heat is applied. If the temperature difference betweenthe source and the soil and the time interval are both small then the amount of moisture movement will also be small. The temperature of the source (size of current) and time interval were so chosen that the amount of moisture movement can be considered negligible. As the sensitivity of the apparatus is roughly ohm adjustable resistance 41. In this circuit is 60 of this wire 51 can be engaged by a switch 5| though it is possible to momentarily close the circuit for determining the zero setting of the bridge. When this setting is once accomplished, then the switch 53 can beclosed while the balancing circuit is left out of circuit; then the reading can be taken to indicate the moisture content of the material in terms of heat conductance or heat capacity as indicated on the microammeter.

Method of operation In operating the apparatus of this invention according to our method we perform thefollowproportional to the cube of the current through the bridge, it is important that the current be kept constant during the progress of a determination.

With reference to Figures Sand '7, this modified circuit is arranged as follows. 25 designates a leg of the Wheatstone bridge to one terminal of which at 26- is attached to the wire 5a of the coil 2. The other terminal of the wire In of the coil 2 is attached to the terminal 21 of the crosswire 28 containing the microammeter 29. An auxiliary lead lb is attached to the coil 2 for the purpose of compensating for temperature fluctuations along the leads 5a, la and lb. This is particularly valuable when long leads are necessary. The leg 30 contains the 1 ohm resistance 3|, the 5 ohm resistance 32 and the 30 ohm resistance 33. The leg 34 contains the variable resistance 35. consisting of 30 ohm vari- 'able resistance 36 anda 30 ohm resistance 31 in series which are in turn in parallel with the adjustable 15 ohm resistance 38. Also interposed in the leg 34 are the parallel resistance 39 which is the 30 ohm resistance and a 200ohm resistance 40. The leg 4| contains a 2 ohm resistance 42 and a 5 ohm resistance 43. The terminal 44has connected to it the wire 45 in which is a half ohm adjustable resistance 46 and a 20 ing steps: I

(a) We insert the coil 2 in the material the moisture in which is to be measured or we pass such material through the interior of the coil by utilizing a structure such as shown in Figure 4.

If the latter structure is used the material may bepassed continuously through the passageway within the coil 2. I

(b) Then the bridge is balanced with a small current from the balancing or calibrating circuit by the closing of the switch in the balancing circuitand thereby supplying a small amount ofcurrent until the microammeter reads zero. The

instrument is adjusted until it so reads.

(0) After reaching a zero balance then clos the switch again and pass the full current for a substantial period of approximately one minute. Then read the microammeter deflection which is an index of the moisture content in accordance with the previous calibration originally made as to the amount of moisture in the material being measured which will give such a reading. If the medium being tested is moving, as in the case of a body of such material passing through the inside of coil 2, then the circuit can be kept closed and the variations in moisture content of the medium read on a microammeter.

It will be thus noted'that the heatconductivity of the material being'measured is a function of its moisture content, i. e., the heat conductivity varies with the moisture. 0r, stated in another way, the variation in moisture content will vary with the heat conductivity or heat capacity.

In operating any one of these circuits and mechanisms in accordance with our method, it is desirable to first bring -the circuit to a zero reading and then again close the circuit in order to read the moisture content.

In balancing the bridge a ,small current of about 0.02 ampere is used so that the heat developed in coil 2 can be dissipated in the soil without causing the temperature to rise. (The circuit is made and broken immediately.) Resistance I2 is adjusted until no current flows through the microammeter. The bridge remains in balance as long as the temperature of'coil 2 remains constant. When the temperature of the soil changes, a new value of resistance I2 is obtained at the balanced condition. Thus, it can be seen that the value of coil 2 at balance is an index of the soil temperature; in fact, the resistance box could be calibrated in degrees, it desired. f

It a current of 0.1 ampere or more is allowed to flow through the bridge continuously after a balance has been obtained, the bridge is thrown out of balance, thus allowing a current to flow through the microammeter. This is due to the conductivity of the soil is the factor that controls the temperature rise. If the conductivity is low the temperature rise will be large, and if the conductivity is high the temperature rise will be small. Since the heat conductivity of a soil increases with its moisture content, the size of the current through the microammeter is an index of the amount of moisture in the soil. It is not necessary to keep the current running until the equilibrium condition has been reached. A microammeter reading taken at any arbitrary time after the current has been turned on serves equally as well.

It will be understood that we desire to comprehend within our invention such modifications as come within the scope of the claims and the invention.

It will be further understood that there exists a number of partial conductors whose resistances are functions of temperature. It is understood that we comprehend within our invention the use of these thermal resistors and that instead of measuring the change in current passing through two arms of a bridge, a change in current flowing through a thermistor may be a suitable device for measuring moisture content.

A thermistor of suitable character has been described in the Bell Laboratories Record for July, 1940, vol. XVIII, No. 11 at page 325. The material of the thermistor may be constituted of silicon carbide, but Wev prefer to use uranium oxide or uranium sulphide. A. thermistor is characterizedby a rapid increase in resistance with temperature.

It is further understood that, since the rate of evaporation of water from a vessel depends upon the relative humidity of the atmosphere, current Our invention is adaptable to such measurements. I

It will be further understood that we comprehend within our invention the possibility of varying a resistance to compensate for the unbalance in a bridge and measuring the amount of unbalance by a deflecting instrument.

Having thus fully described our invention, what we claim as new and desire to secure by Letters Patent is:

1. In a heat conductivity-moisture determining apparatus, a Wheatstone bridge circuit having a heating coil in one leg thereof adapted to be inserted into the material the moisture content of which is to be measured, two other legs of the bridge circuit being formed entirely of fixed resistors, a variable resistance in the fourth leg of the bridge; a microammeter connected across said legs; a current-applying circuit having an ammeter, battery and variable resistances therein, a switch for connecting. said current-applying circuit to said Wheatstone bridge circuit, a supplementary balancing resistance, a switch for selectively connecting said supplementary resistance into or out of series with said current applying circuit, whereby the bridge may be balanced ,with a small current when the supplementary flowing through a resistance coil in the base of a r shallow vessel containing water may serve as a humidostat, and that the magnitude of the cur rent will be a function of the rate of evaporation of the water from the vessei, which in turn, will depend upon the relative humidity.

resistance is connected in series with said current applying circuit.

2. In an apparatus for measuring heat conductivity, the combination, asource of current, a Wheatstone bridge and current adjusting means whereby current supplied to the Wheatstone bridge may be measured both before and after a test, a coil resistor, means mounting said coil resistor including a cylindrical element of insulating material, said coil being connected into one leg of said Wheatstone bridge and adapted to be inserted into the material to be tested, said coil resistor engaging the surface of said cylindrical element whereby when said cylindrical element is inserted into the material to be tested the turns of said coil may be in contact with saidmaterial to be tested; a circuit connected to said'bridge having an ammeter and variable resistance therein. said source of current being also in said circuit, a switch in said lastnamed circuit, and a supplementary balancing resistance, a. switch for selectively connecting said supplementary resistance into and out of series with said last-named circuit, whereby the bridge may be balanced with a small amount of current when the supplementary resistance is connected in series with said last-named circuit.

LEONARD D. BAVER. BYRON T. SHAW. 

